This invention relates to apparatus for storing a predetermined amount of electrical energy in a capacitor bank for discharge through a strobe lamp and, more particularly, to the storage of that electrical energy at a constant rate of energy transfer to the capacitor bank.
Strobe lamps typically comprise a glass bulb in which a gas is sealed which gas when ionized generates high intensity light. The glass bulb typically includes two power electrodes and may include a trigger electrode or may be triggered by external circuitry such as a high voltage transformer. Energy for activating the lamp or ionizing the gas stored within the lamp bulb is generally stored in a bank of capacitors connected in parallel with the lamp. Once a sufficient amount of energy has been stored in the bank of capacitors and it is desired to flash the lamp, a firing pulse is applied to the trigger electrode or external triggering circuitry. The firing pulse ionizes the gas sufficiently to allow current to flow between the two power electrodes which results in the brilliant flash of the strobe lamp.
In the majority of strobe light applications, it is desirable that the strobe lamp light intensity remain above a specified value and approximately constant throughout the operating life of the strobe light. Unfortunately, capacitor banks used in strobe light systems change significantly in capacitance value, both as a result of aging and as a result of variations in ambient temperature. Such capacitance changes are particularly notable in aircraft where severe changes in ambient temperature are routinely encountered. If no correction is made for the variations in capacitance of the capacitor banks, large variations in the amount of energy delivered to the strobe lamp occur over temperature changes and with aging of the strobe light system. Such variations can reduce the life of the strobe light system when excessive energy is provided to the lamp and result in insufficient light intensity at other times.
Various systems have been used to control the energy stored in the capacitor bank. In one such system, the energy stored in the capacitor bank is approximated as a function of the voltage across the bank. Capacitor bank voltage is monitored and compared to a set voltage level to determine when a desired amount of energy has been stored in the bank. A device using this technique is shown in U.S. Pat. No. 3,868,562, issued Feb. 25, 1975. Obviously, such an approach is unacceptable where large capacitance variations in the capacitor bank occur such as in aircraft strobe light systems.
An improved constant energy strobe light system is shown in U.S. Pat. No. 4,005,337, issued Jan. 25, 1977. In this system, the current applied to a capacitor bank is monitored and a signal proportional to that current is integrated and compared to the voltage across the capacitor bank. When the integrated signal equals the voltage across the capacitor bank, the charging of the capacitor bank is interrupted and the energy stored within the capacitor bank can then be discharged through a strobe lamp. In accordance with this technique, equal amounts of energy are applied to the capacitor bank in preparation for each strobe lamp discharge.
While the constant energy strobe source is a great improvement over the technique disclosed in the earlier patent, both of these prior art techniques result in high current levels during energy transfer to the capacitor bank. Not only do such high current levels require high rated circuit devices but they can also lead to higher levels of electromagnetic interference (EMI) generated by the strobe light system. Additionally, in the prior art strobe light systems, if one or more capacitors in the capacitor bank fail and have to be replaced, the prior art units must be reset to ensure proper operation.